When I was 17 (over thirty years ago) I had my first rifle built, it was a sporterized 98 Mauser originally an 8X57 with the stepped military barrel. I had it re-barreled with a Shilen in .257 Roberts Ackley Improved, due to lack of funds (and knowledge) I kept the white pine ell-cheapo stock.
Had many many years of fun with this gun, lots of deer, varmints and targets shot, even one elk. I seem to be afflicted with some wierd disease wherein I am unable to recall any but the best groups I have ever shot, with this gun that is about 1/2 MOA out to 300 yards.
I give the background in order to explain that this rifle has nostalgic/sentimental appeal to me. I recently decided to give it a face lift and try to squeeze some more accuracy out of it. I sent the barreled action to a gunsmith to true it up, he lapped the lugs, bolt face and trued the action, then I pillar bedded it in a laminated stock.
In the process of fire forming some new brass, I noticed that the fired cases are not round, I am measuring about 0.003" to 0.004" out of round at the pressure ring on the case. I checked some of my old brass and found the same thing, I guess I just never looked that close before and so did not notice it until now.
Now, after the long winded introduction comes the question:
Can this chamber be re-reamed and trued up or will the reamer just follow an already out of round chamber and end up not in line with the bore?
By the way, I would like to set the barrel back about 0.250" anyway so that I can seat 100 Gr. bullets out to the lands. The barrel always was set too far out and with whatever erosion has taken place it would not hurt to set it back a ways.
If your going to set it back, have the thread tennon removed and recut. This will give the smith more room to correct the error, and only remove about 5/8 of an inch of barrel. Now as far as making the new chamber concentric with the bore, that will require the proper set-up and indication of the bore at the throat. Then the old chamber should be bored concentric with that set-up so the reamer is not influenced by the old "out" chamber. If your goal is this explain that to your smith, or he might just run the reamer in and follow what you had to begin with.
My best advice is don't stick any money into a 30 year old tube for the sake of keeping brass concentric. If you go thru the above, you might as well add a few hundred for a new tube.
However, What I've done with in a similar situation there was sentimental attraction in one case and the rifle was worthless in the other was to prior to firing the cartridge was to file a mark on the head and insert the cartridge with that mark up.
Then keep that mark up in all cases in the future, when neck sizing only.
This may work since you have had great success with the past use of the rifle.
We know most action are fail tested to pressures considerably higher than factory loadings. Knowing that the mauser was invented near the turn of the later century and used the very first smokless powders invented at the time, and understanding the original 8mm or 7mm for that matter, were a bit anemic in the loadings that were put thru the original Mausers.
I understand that plenty of mausers were chambered in the conventional magnums, and I have also seen some mausers with set-back lug abuments due to a lack of a proper heat treat, or pressures/ bolt thrust that created this over its usefull life. I have also seen some home shop specials that were welded after the front reciever ring apeared to have cracked after barrel removal. If your buying a Mauser look very carefully at them for any signs of tampering/rework. I dam near barreled that action before I noticed the faint seam inside the thread tennon, I give the welder credit he did a fine job with the polishing and cold blue. (this was one of the reasons I swore off working on mititary action)
The thread tennon on a mauser is 1" and with as coarse a thread that is cut on a mauser this does not leave a great abount of wall thickness between the thread root and chamber wall on a case that is .550 inches in dia. Add the fact that the UM class cartriges are run at higher pressures and I would advise against it in that action. I'm sure someone has done it but I wouldn't.
For liability reasons some of the smiths that specialize in rebuilding Military Mausers send them out for a proper heat treat before re barreling. Probably a good idea since some of them were built with prisoner labor. As far as I know this is not an issue with any of the comercial mauser clone actions.
a little long winded but might help some others who read this post as well.
Concerning sentiment......... My dad's old REM 721 '06 that I shot my first (and only PA buck with) now sports a 270 AM barrel. Thus I split the difference between sentiment and a great shootin' long range rig.